As is well known, the semiconductor technology has made remarkable advances toward higher integration of integrated circuits. This tendency promoted to use a light source of shorter wavelength in the lithography process for semiconductor device manufacture. Photolithography using ArF excimer laser (193 nm) is the current main stream. A transition to photolithography using extreme ultraviolet (EUV) is expected to enable further integration. As the technology for the fabrication of semiconductor devices with a half-pitch of 32 nm or less, not only the photolithography, but also the nanoimprint lithography are considered promising.
The nanoimprint lithography is expected to find a wide variety of applications including optical waveguides, bio-chips, and optical storage media.
The nanoimprint lithography involves furnishing a mold (also referred to as stamp or template) having a fine pattern predefined thereon by electron beam lithography and etching techniques, coating a resin material on a substrate, and forcing the mold against the resin film for transferring the configuration of the fine pattern to the resin film. Specifically, semiconductor devices are fabricated by forcing a mold against a resist film coated on the surface of semiconductor wafer such as silicon for transferring the fine pattern.
The nanoimprint lithography is generally divided into photo nanoimprint lithography and thermal nanoimprint lithography. The photo nanoimprint lithography uses a photo-curable resin as the resin material. While the mold is pressed against the resin, ultraviolet (UV) radiation is irradiated to the resin for curing, thereby transferring a fine pattern.
On the other hand, the thermal nanoimprint lithography uses a thermoplastic resin as the resin material. A fine pattern is transferred by pressing the mold against the thermoplastic resin which has been softened by heating above the glass transition temperature. Alternatively, a fine pattern is transferred by pressing the mold against a thermosetting resin while heating up to the curing temperature.
The properties required for nanoimprint molds include a mechanical strength to prevent failure of the mold during fine pattern transfer and a chemical stability to be inert to the resin.
The thermal nanoimprint lithography has to apply heat for the purpose of softening thermoplastic resins or curing thermosetting resins. The mold used therein experiences a temperature change from room temperature to about 200° C., depending on the type of resin used. If the mold is made of a material having thermal expansion, any deformation of the mold can bring about a decline of location accuracy. Then the mold used in the thermal nanoimprint lithography is desirably made of a material having minimal thermal expansion over the wide temperature range from room temperature to about 200° C.
In the photo nanoimprint lithography, the mold does not experience a temperature change as occurring in the thermal nanoimprint lithography. Then only thermal expansion around room temperature is of concern. However, since the photo nanoimprint lithography is expected applicable to the fabrication of semiconductor devices with a fine feature size of 32 nm or less, a more strict location accuracy is needed. Also to enable single large-area transfer which is one of the advantages of the photo nanoimprint lithography, the mold must be made of a material having a uniform thermal expansion as well as low thermal expansion around room temperature.
JP-A 2006-306674 discloses the use of a low thermal expansion material as the mold material in the photo nanoimprint lithography. The more precise transfer of a fine pattern, however, requires to control the distribution of a coefficient of linear thermal expansion within the mold. Also the thermal nanoimprint lithography requires a mold material having low thermal expansion over a wider temperature range.